A simulation study of the degrees of freedom of movement in reaching and grasping

The question of independently controlled components in the act of reaching and grasping has attracted interest experimentally and theoretically. Data from 35 studies were recently found consistent with simulated kinematic finger and thumb trajectories optimised for minimum jerk. The present study closely reproduces those trajectories using a discrete-time model based on minimum acceleration. That model was further used to generate two-dimensional trajectories for finger and thumb to reach and grasp an elliptical object with varying position and/or orientation. Orthogonalisation of these four trajectories revealed one degree of freedom when direction of reach was constant and two degrees of freedom when direction of reach varied, irrespective of object distance and orientation. These simulations indicate that reach and grasp movements contain redundancy that is removable by formation of task-dependent synergies. As skilled movement can be planned and executed in a low dimension workspace, control of these independent components lessens central workload.     

A new view on visuomotor channels: the case of the disappearing dynamics

A considerable body of kinematic data supports the proposal that independent visuomotor channels are involved in the control of the transport and grip components of reach and grasp. These channels are seen as having separate perceptual inputs, outputs and internal processing and are thought by some to correspond to independent neuroanatomical pathways. The idea that different groups of muscles and biomechanical structures can be controlled independently is attractive, but this kinematically-inspired hypothesis fails to take into account the complexity of the dynamic relationships and their interactions within the neuromusculoskeletal system. Inertial, viscous, centrifugal, coriolis, gravitational and reflex cross couplings exist between efferent drives to muscles and resulting body movements. Rotation at even a single joint generates a complex set of dynamic reaction forces and requires coordinated activation of many muscles throughout the body to maintain posture and balance. In this theoretical paper we present a new view of independent visuomotor channels in the form of an adaptive neural controller that can compensate for the above interactions and decouple the relationships between efferent drives to muscles and resulting body movements. At the same time, the neural controller renders all the dynamics (linear and nonlinear), other than time delays, of the neuromusculoskeletal system, unobservable in the visuomotor relationships. Using the geometry of nonlinear dynamical systems we show that, providing certain constraints on the structure of time delays within the system are satisfied, there exists a neural controller that can render all the dynamics of the neuromusculoskeletal system (except for time delays) unobservable in the responses. The controller simultaneously decouples all the interactive dynamics so that each of the m independent inputs controls one and only one degree of freedom of the response. This means that each degree of freedom in a multi-joint response can be controlled by an independent component of the visual input, a behaviour that has long been observed in visual tracking experiments. The controller effectively establishes m independent visuomotor channels. However, rather than reflecting separate neuroanatomical pathways, the independent channels result from a neural controller with convergent and divergent connections to compensate for the interactive nonlinear dynamics within the neuromusculoskeletal system. This new view of visuomotor channels has implications for neural control processes involved in the acquisition and adaptability of skilled perceptual-motor behaviour in general, as well as for the design of robotic controllers.     

Boycott or buycott? Understanding political consumerism

• This research addresses the question of how boycotting (punishing business for unfavorable behavior) differs from buycotting (rewarding business for favorable behavior). This analysis of 21 535 adults from the 2002/2003 European Social Survey (ESS) compares the effects of social capital, altruism, and gender on different categories of political consumers. Logistic regression analyses reveal that boycotters do indeed differ from buycotters. Specifically, women and people who are more trusting, involved in more voluntary associations, or more altruistic are more likely to buycott than boycott. These differences support the inclusion of both boycott and buycott measures in future studies of political consumerism.

Copyright © 2010 John Wiley & Sons, Ltd.     

Anisotropic tracking: evidence for automatic synergy formation in a bimanual task

Investigation of interlimb synergy has become synonymous with the study of coordination dynamics and is largely confined to periodic movement. Based on a computational approach this paper demonstrates a method of investigating the formation of a novel synergy in the context of stochastic, spatially asymmetric movements. Nine right-handed participants performed a two degrees of freedom (2D) "etch-a-sketch" tracking task where the right hand controlled the horizontal position of the response cursor on the display while the left hand controlled the vertical position. In a pre-practice 2D tracking task, measures of phase lag between the irregularly moving target and the response showed that participants controlled left and right hands independently, performance of the right hand being slightly superior to the left. Participants then undertook 4 h 16 min distributed practice of a one degree of freedom etch-a-sketch task where the target was constrained to move irregularly in only the 45 degrees direction on the display. To track such a target accurately participants had to make in-phase coupled stochastic movements of the hands. In a post-practice 2D task, measures of phase lag showed anisotropic improvement in performance, the amount of improvement depending on the direction of motion on the display. Improvement was greatest in the practised 45 degrees and least in the orthogonal 135 degrees direction. Best and worst performances were no longer in the directions associated with right and left hands independently, but in directions requiring coupled movements of the two hands. These data support the proposal that the nervous system can establish a model of novel coupling between the hands and thereby form a task-dependent bimanual synergy for controlling the stochastic coupled movements as an entity.     

A Patient's Perspective on Genetic Counseling and Predictive Testing for Alzheimer's Disease

The availability of genetic testing for Alzheimer's disease is anticipated to be widespread in the future. As an individual at risk with a family history of Alzheimer's disease, I discuss why I sought predictive tests and how I would use the information from such tests. I relay what I learned in my genetic counseling session, my response to the counseling process, and steps I have since taken. I discuss life planning, psychological and fear of discrimination issues from a patient's perspective.     

Berkeleian Principles in Ecological Realism: An Ontological Analysis

Three experiments are reported, which examined the relation between the percep- tion of the distance reachable with a hand-held rod that can be wielded but not seen and the rod's resistance to having its rotational speed changed by application of a torque. In these experiments, subjects wielded any given rod about an axis intermediate between its endpoints. The subject's task was to adjust a visible, movable surface to coincide with where he or she could reach with the given rod if allowed to hold it at its proximal end. Two experiments considered the effects of wielding a rod at different orientations to the pull of gravity. Rods were wielded either within a plane roughly perpendicular to the ground or within a plane roughly parallel to the ground. Plane of wielding did not affect the patterning of perceived reachable distances as a function of the various conditions, which included variations in the positioning of grasp and the positioning of a mass affixed to the rods. The patterning of the moments of inertia associated with the various conditions determined the patterning of perceived reachable distances. The third experiment restricted wielding to a plane roughly parallel to the ground and varied how the rods were grasped, either overhand or underhand. The variation in grasp amounted to a variation in the neuromuscular patterning associated with the wielding of any given rod. Perceived reachable distances proved to be indifferent to the overhand versus underhand contrast.     

Comorbid Problems in ADHD: Degree of Association,Shared Endophenotypes,and Formation of Distinct Subtypes. Implications for a Future <Emphasis Type="Italic">DSM</Emphasis>

We aimed to assess which comorbid problems (oppositional defiant behaviors, anxiety, autistic traits, motor coordination problems, and reading problems) were most associated with Attention-Deficit/Hyperactivity Disorder (ADHD); to determine whether these comorbid problems shared executive and motor problems on an endophenotype level with ADHD; and to determine whether executive functioning (EF)—and motor-endophenotypes supported the hypothesis that ADHD with comorbid problems is a qualitatively different phenotype than ADHD without comorbid problems. An EF—and a motor-endophenotype were formed based on nine neuropsychological tasks administered to 816 children from ADHD—and control-families. Additional data on comorbid problems were gathered using questionnaires. Results indicated that oppositional defiant behaviors appeared the most important comorbid problems of ADHD, followed by autistic traits, and than followed by motor coordination problems, anxiety, and reading problems. Both the EF—and motor-endophenotype were correlated and cross-correlated in siblings to autistic traits, motor coordination problems and reading problems, suggesting ADHD and these comorbid problems may possibly share familial/genetic EF and motor deficits. No such results were found for oppositional defiant behaviors and anxiety. ADHD in co-occurrence with comorbid problems may not be best seen as a distinct subtype of ADHD, but further research is warranted. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Degrees of freedom and motor planning in purposive movement

This paper presents empirical evidence suggesting that healthy humans can perform a two degree of freedom visuo-motor pursuit tracking task with the same response time delay as a one degree of freedom task. In contrast, the time delay of the response is influenced markedly by the nature of the motor synergy required to produce it. We suggest a conceptual account of this evidence based on adaptive model theory, which combines theories of intermittency from psychology and adaptive optimal control from engineering. The intermittent response planning stage has a fixed period. It possesses multiple optimal trajectory generators such that multiple degrees of freedom can be planned concurrently, without requiring an increase in the planning period. In tasks which require unfamiliar motor synergies, or are deemed to be incompatible, internal adaptive models representing movement dynamics are inaccurate. This means that the actual response which is produced will deviate from the one which is planned. For a given target-response discrepancy, corrective response trajectories of longer duration are planned, consistent with the principle of speed-accuracy trade-off. Compared to familiar or compatible tasks, this results in a longer response time delay and reduced accuracy. From the standpoint of the intermittency approach, the findings of this study help make possible a more integral and predictive account of purposive action.     

The BUMP model of response planning: variable horizon predictive control accounts for the speed-accuracy tradeoffs and velocity profiles of aimed movement

The BUMP model is a comprehensive discrete-time computational model of response planning. Developed within the Adaptive Model Theory framework, it is based on intermittent optimal control. The theory posits a basic unit of motor production (BUMP) that is determined by a planning system that operates intermittently at fixed intervals of time. Given sensory information about the position and velocity of the actual response as well as the predicted future state of the target, the response planning system generates an optimal response trajectory to reach the predicted future state of the target and to compensate for executional error. The ability to vary the duration, or prediction horizon, of the trajectory gives rise to the concept of variable horizon predictive control. We propose that the combination of signal-dependent noise in the nervous system and variable horizon predictive control accounts for the well-known speed-accuracy tradeoffs and velocity profiles in aimed movements. Conducting a simulation study, we found that on one extreme of variable horizon control, a receding horizon strategy reproduced Fitts' law and corresponding asymmetrical velocity profiles. On the other extreme, a fixed horizon strategy reproduced the linear tradeoff and corresponding symmetrical velocity profiles. We conclude that the BUMP model provides a unifying theoretical bridge between speed-accuracy tradeoffs and the accompanying velocity profiles of aimed movement.